1. Field of the Invention
It is known that to achieve a good self-cleaning effect of a surface, the surface must have a microrough surface structure as well as good hydrophobicity. Both features are realized in nature, for example, in the lotus leaf. The surface formed from a hydrophobic material has relatively large spherical elevations, which can be described as studs, which sit so close to one another that a water drop does not fit between them and thus only sits on the tips of the studs. Besides these relatively large elevations, both the surface of the rest of the lotus leaf and also the surface of the relatively large elevations themselves are in turn substructured by relatively small elements, in the case of the lotus leaf, wax particles. Overall, this gives a structure which consists of elevations and depressions of from sometimes less than 100 nm ranging to 30 μm.
2. Description of the Related Art
Water-repellant surfaces with a microrough structure with elevations and depressions are described in U.S. Pat. No. 3,354,022 with processing here being with a hydrophobic material, in particular a fluorine-containing polymer. According to one embodiment, a surface with self-cleaning effect can be applied to ceramic tiles or to glass by coating the substrate with a suspension which comprises glass beads with a diameter in the range from 3 to 12 μm and a fluorocarbon wax based on a fluoroalkylethoxy-methacrylate polymer. Disadvantages of such coatings are their low abrasion resistance and moderate self-cleaning effect.
The laid-open specification EP 0 909 747 A1 teaches a method for producing a self-cleaning property of surfaces, in particular roof tiles. The surface has hydrophobic elevations with a height of from 5 to 200 μm. A surface of this type is produced by applying a dispersion of powder particles from an inert material in a siloxane solution and subsequent curing. As in the previously acknowledged method, the structure-forming particles are not fixed to the surface of the substrate in an abrasion-stable manner.
EP patent 0 772 514 teaches self-cleaning surfaces of objects with an artificial surface structure of elevations and depressions, where the distance between the elevations is in the range from 5 mm to 200 mm and the height of the elevations is in the range from 5 mm to 100 mm and the structure consists of hydrophobic polymers or lastingly hydrophobized materials. Suitable for forming the structure are etching and embossing methods, and also coating methods such as adhering a hydrophobic polymer. If required, structure formation is followed by hydrophobization, for example a silanization. The self-cleaning surfaces, such as coatings of vehicles, i.e. surfaces to be cleaned by readily moved water, must not be subjected to strong mechanical stresses since this causes the ability for self-cleaning to be lost.
The DE application DE 100 63 739 teaches substrates such as glass, ceramic, plastic and metal, as well as glazed and enameled substrates, with a self-cleaning surface. The self-cleaning surface comprises structure-forming particles with a mean diameter of less than 100 nm, in particular less than 50 nm, but at least 5 nm and a layer-forming material which is an inorganic or organic material. The structure-forming particles are fixed to the substrate by means of the layer-forming material. Some of the primary particles and/or agglomerates of the same protrude at least partially from the surface either themselves, or coated with layer-forming material, and in so doing form elevations and depressions in the nanoscale range. The structured surface has a hydrophobic coating at least in parts. To produce these types of self-cleaning surfaces, a composition which comprises structure-forming particles and an organic or inorganic layer-forming material is applied to the substrate by means of known coating methods, for example a lacquer. The formation of a coherent and firmly adhering layer by a thermal treatment is followed by a hydrophobization, e.g. using fluorine-containing silanes and/or fluorine-containing siloxanes. The formation of the surface structure and the hydrophobization take place in this method in separate steps.
WO 02/064266 describes a self-cleaning paint coating comprising a base paint coat and a topcoat with an artificial surface structure, formed from particles, of elevations and depressions, wherein the particles have an average particle diameter of less than 100 nm, are bonded at least partially by means of a binder system in the topcoat, the mean height and the mean distance between the elevations are less than 50 nm, and the surface or topcoat is at least partially hydrophobic. The disclosed self-cleaning paint coating has a nanoscale surface structure with hydrophobic character. Since the structure-forming primary particles, with diameters of <100 nm, preferably <50 nm to about 5 nm, can also form agglomerates, the surface structure apart from the specified elevations and depressions can also exhibit a superstructure with greater distances and heights. In general, the average heights and distances of the elevations of the superstructure are still significantly below 1 μm. The structure-imparting particles may be organic or inorganic materials.
DE 102 33 829 A1 describes a method for generating a lotus structure by applying the particles in dry form by electrostatic spraying by means of powder spray guns.
It is common to all of these inventions that they either describe a coarse structure with structural elements in the μm range or deal with very small nanoscale structural elements which produce corresponding fine structures. A combination of a structure that is at least a few μm in size which a very fine nanoscale substructure covered with structural elements which are smaller than 500 nm overlies is not known. However, precisely this surface structure of a combination of relatively large structural elements which a smaller-dimensioned substructure overlies, is the characteristic surface structure of the lotus leaf surface. In particular, this fine structure in the case of the lotus leaf is present both on the leaf surface which lies between the relatively large elevations, and also on the larger elevations which covers them to a certain extent. By combining relatively small and relatively large particles, although it is possible to produce a combination of relatively small and relatively large elevations, the relatively small particles collect in the gaps between the relatively large ones, as a result of which the packing density increases, and do not cover the relatively large particles. This technical principle is utilized in order to increase the packing density in coatings by combining large and small particles.
Similarly structured surfaces with hydrophobic properties are taught in EP 0 933 388 A2. The surface has elevations with a mean height of from 50 nm to 10 μm and a mean distance between 50 nm and 10 μm, and also a surface energy of the unstructured material of from 10 to 20 mN/m. To achieve a particularly low surface energy and thus hydrophobic and oleophobic properties, the structured surface has fluorine-containing polymers or it has been treated using alkylfluorosilanes.
A combination, as described above, of a coarse structure which is coated in a targeted manner by a fine structure is not described here either.
EP 0 433 727 A1 describes silicon-based core-shell particles in which the core consists of colloidal silica and the shell of siloxanes or siloxane condensates. The particle size is between 4 and 400 nm.
US 2003/0044612 A1 claims core-shell particles which comprise a porous core of an organic polymer and a shell of inorganic colloidal material, preferably colloidal silica.
A belief has developed that a targeted fine structuring of a coarsely structured surface can only be achieved through the combination of fine particles and relatively large particles, where the relatively large particles must bring with them the fine structure which can be achieved by the smaller particles on their own, already firmly anchored to their surface. This is because, as a result of subsequent application of a coating material which comprises the smaller particles to a coarsely structured surface, the phenomenon described above always arises to a certain degree, that the relatively small particles do not accumulate on the tips of the raised areas of the surface structure, but in between them, for which reason inhomogeneous distribution of the fractions that impart a fine structure is unavoidable.